Airflow direction indicators

ABSTRACT

An airflow direction indicator has a hollow shaft mounted for sealed rotation into an enclosure and supporting an inner shaft carrying a vane which is located in the airstream. A seal is provided between the inner shaft and the hollow shaft which allows their relative rotation through a limited angle. A servo system is provided which operates to maintain the hollow shaft and the inner shaft within the limited angle relative to one another, and output means indicate the angular position of the hollow shaft relative to a datum position.

ilited States Patent Pitches et al.

[ 51 May 30, 1972 AIRF LOW DIRECTION INDICATORS inventors: Brian EdwardPitches, Balerno; Robert Malcolm Stuart Murray, Edinburgh, both ofScotland Assignee: Ferranti, Limited, Hollinwood, Lancashire, EnglandFiled: Dec. 14, 1970 App]. No.: 97,927

Foreign Application Priority Data Dec. 15, I969 Great Britain..60,9l8/69 US. Cl

Int. Cl Field of Search ..73/1 80, 188, 189

[56] References Cited UNITED STATES PATENTS 3,105,382 l0/l963 Friedmanet al ..73/l80 Primary Examiner-Donald O. Woodiel Attorney-Cameron,Kerkam & Sutton 57 ABSTRACT An airflow direction indicator has a hollowshaft mounted for sealed rotation into an enclosure and supporting aninner shaft carrying a vane which is located in the airstream. A seal isprovided between the inner shaft and the hollow shaft which allows theirrelative rotation through a limited angle. A servo system is providedwhich operates to maintain the hollow shaft and the inner shaft withinthe limited angle relative to one another, and output means indicate theangular position of the hollow shaft relative to a datum position.

4 Claims, 4 Drawing Figures Patented May 30, 1972 3 Sheets-Sheet 1Patented May 30, 1972 3 Sheets-Sheet 2 Patented May 30, 1972 3,665,760

3 Sheets-Shut S AIRFLOW DIRECTION INDICATORS This invention relates toan airflow direction indicator primarily, though not exclusively, foruse as an aircraft angleof-attack sensor.

Airflow direction indicators in general fall into several distinctclasses in dependence upon their mode of operation, and one of theseclasses involves locating a rotatable vane in the airstream andmeasuring the angular position of the vane relative to a datum. One ofthe problems with devices of this type has been the necessity to allowfreedom of rotation of the vane by the airstream while providing aneffective seal against the ingress of moisture or dust into themechanism. It has been found that effective seals have introducedfrictional forces opposing the movement of the vane and henceintroducing errors into the output indication given by theindicator.

Also with airflow direction indicators of the type referred to above,difficulty has been experienced by the tendency of the vane to respondin an oscillatory manner to changes of direction of the airstream.Simple damping arrangements, such as viscous damping of the shaftcarrying the vane, are not equally effective over the whole range ofairspeeds since the aerodynamic forces acting on the vane areproportional to the square of the airspeed. Hence a compromise has to bemade in which the vane is underdamped at high airspeeds and overdampedat low airspeeds, with the result that the response of the instrumentvaries over the range of airspeeds.

It is an object of the invention to provide an airflow directionindicator incorporating a rotatable vane, which may be effectivelysealed without introducing serious errors due to friction.

It is a further object of the invention to provide an airflow directionindicator in which movement of the vane is itself undamped but in whichoscillations of the vane are damped out and do not appear in the outputindication.

According to the present invention there is provided an airflowdirection indicator which includes a hollow shaft mounted for sealedrotation into and with respect to a supporting enclosure, an inner shaftpassing coaxially through the hollow shaft for rotation relativethereto, a vane arranged for location in the airstream and rigidlyattached to the inner shaft, a seal between the inner shaft and thehollow shaft'arranged to allow their relative rotation through a limitedangle, servo control means operable to rotate the hollow shaft tomaintain the relative positions of the inner shaft and the ho]- lowshaft within said limited angle, and output means operable to indicatethe angular position of the hollow shaft relative to a datum position.

The invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a part-sectioned perspective view of an airflow directionindicator according to one embodiment of the invention;

FIG. 2 is a sectional side view of the indicator of FIG. 1;

FIG. 3 is a sectional end view of the indicator along the line llllII ofFIG. 2; and

FIG. 4 is a schematic diagram illustrating the operation of theindicator.

Referring now to FIGS. 1 to 3, a'cylindrical housing is located by meansof a flange l l in an aperture in the fuselage 12 of an aircraft. Thehousing 10 supports two sleeve bearings 13 in which is carried a hollowshaft 14. The hollow shaft itself carries two ball races 15 in which iscarried an inner shaft 16 passing through the hollow shaft 14. Theoutboard end of the inner shaft 16 has secured to it a cap 17 carrying avane 18 shown in the form of a double wedge supersonic aerofoil section,swept back at 45 to the axis of rotation. The inboard end of the innershaft 16 carries a balance weight 19 attached by an arm 20 to a collar21.

Around the hollow shaft 14 is fastened a gear-ring 22, the teeth ofwhich engage with a pinion 23 on the spindle of an electric motor 24having a gear-box 25, and another pinion 26 on the spindle of atacho-generator 27. The motor 24 and the tachogenerator 27 are carriedin the housing 10.

The inboard end of the hollow shaft 14 carries a pair of lugs 28 and 29,projecting radially outwards from the shaft and arranged facing butspaced apart from one another. Each lug has an insulated electricalcontact 30 projecting through it into the space between the two lugs.The arm 20 securing the balance weight 19 to the inner shaft 16 carriesa projecting finger 31 arranged to be located between the contacts 30 onthe two lugs 28 and 29. Electrical connections will be taken from thetwo contacts 30 and from a contact surface on the finger 31, throughslip rings or flexible leads (not shown) for connection to theassociated equipment.

A dust and moisture-proof seal 32 is secured to the outboard ends of thehollow shaft 14 and the inner shaft 16, allowing only limited relativerotation between them, and is partly covered by the cap 17.

FIG. 4 illustrates the servo control means and output means of theairflow direction indicator described above. The indicator itself isshown in schematic form only. The hollow shaft 14 carrying electricalcontacts on two lugs 28 and 29 is shown, together with the inner shaft16 with its contact finger 31. Across the two contacts on the lugs 28and 29 is connected a battery 33, and the contact surface on the finger31 is connected to the input of an amplifier 34. The amplifier outputcontrols the motor 24 which is mechanically coupled through the gearbox25 to the tachogenerator 27 and to the hollow shaft 14. The electricaloutput of the tachogenerator 27 is fed back to the amplifier 34.

An output representing the angular position of the hollow shaft relativeto a datum position may be indicated by the position of a movablecontact 35 on a potentiometer 36, the movable contact being mechanicallyconnected to the hollow shaft 14. The mechanical and electricalarrangements shown in FIG. 4 comprise a conventional follow-up servoloop with mechanical feedback.

The lugs 28 and 29 are arranged so that the inner shaft rotationrelative to the hollow shaft is very small, say a total of 0.2 degreesof arc.

Since there is very little relative movement between the hollow shaft 14and the inner shaft 16, the flexible end seal 32 will accommodate thismovement while preventing the ingress of moisture or dust into theindicator. The sleeve bearings 13 supporting the hollow outer shaft 14in the housing 10 perform the same function. Friction in these bearingsis overcome by the torque available from the motor 24.

The operation of the airflow direction indicator described above is asfollows:

Consider initially that the system has become stable in the airstream,such that the finger 31 on the arm 20 is located mid-way between, andseparated from, the two contacts 30 carried on the lugs 28 and 29. Theamplifier 34 will be ineffective, and hence the motor 24 will bestationary. The position of the slider 35 on potentiometer 36 indicatesthe angular position of the vane 18 relative to the datum direction.

If now the direction of the airflow relative to the aircraft changes,then the vane 18 is caused to rotate. Only slight rotation is possible,until the finger 31 comes against the contact 30 on one of the lugs 28or 29. When, for example, the finger touches contact 30 on lug 29, dueto anticlockwise movement of the inner shaft 16 as viewed in FIG. 3, theamplifier 34 causes the motor 24 to be energized and drive the hollowshaft 16. This shaft is driven in an anticlockwise direction, as viewedin FIG. 3, so long as the finger 31 remains touching the contact on lug29. The tachogenerator 37 feeds back to the amplifier a signalindicating the speed of movement of the hollow shaft 14. The inner shaft16 is now free to be rotated by the effect of the airstream on the vane18, and the hollow shaft 14 will continue to rotate until the vanereaches an equilibrium position. When the inner shaft stops rotating,further movement of the hollow shaft breaks the electrical contactbetween the finger 31 and contact 30 on lug 29. The motor 24 is then nolonger energized by the amplifier 34, and movement of the hollow shaft16 ceases. The slider 35 of potentiometer 36 has been moving with therotation of the hollow shaft, and its new position now indicates theposition of the vane.

In the same way, movement of the vane 18 in the opposite directioncauses the motor to be energized so as to rotate the hollow shaft in thesame direction as the vane until the finger 31 separates from thecontact 30 on lug 28.

Due to the gearing between the hollow shaft 14 and the motor, includingthat within the gearbox 25, it is not possible for aerodynamic forcesapplied to the vane to rotate the hollow shaft due to contact betweenthe finger 31 and one of the lugs 28 or 29. Hence the lugs 28 and 29 actas stops to limit the rotation of the inner shaft 16.

In practice the rotation of the hollow shaft will not stop immediatelyfinger 31 and the appropriate contact 30 separate, and the movement maycontinue until the opposite contact touches the finger. This will causehunting of the servo system, unless movement of the hollow shaft isdamped in some way. A certain amount of damping is provided by thetachogenerator 27. The problem of hunting is due largely to the on-off"type of pick-off used to indicate relative movement between the hollowshaft 14 and the inner shaft 16. It is therefore advantageous to usesome other type of pick-off which will indicate both the direction andthe extent of such relative movement. it is possible to use capacitive,inductive or piezo-electric pick-offs for example, in place of thefinger 31 and contacts 30.

The output potentiometer 36 may be replaced by some other type of outputmeans. For example a synchro may be used to give a remote indication.

Any suitable type of vane may be used, and the shape and dispositionwill depend to a large extent to the possible speed of the aircraft.

An airflow direction indicator used in an aircraft may have a limitedrange of operation, say about 50. However, the indicator may equally beused in applications where a larger angle of operation, up to 360, isrequired. The range will determine whether slip-rings or flexible leadsare used for the connections to the contacts 30 and finger 31.

It will be appreciated that, quite distinct from any tendency of theoutput shaft 14 to oscillate in following the movement of the vane,which tendency may be reduced by normal servodamping techniques such asvelocity feedback from the tachogenerator 27, the vane itself and hencethe inner shaft may tend to respond to changes in the direction of theairflow in an oscillatory manner determined by the inertia of the vaneand the aerodynamic restoring forces. In the airflow direction detectorconstructed as described above, the inner shaft 16 and the vane areundamped and free to oscillate, but only through a very small anglelimited by the stops provided on the lug 28 and 29. Although the vanemay urge the finger 31 quite forcefully into contact with the stops,these forces are reduced at the motor shaft by the action of the gearingas described above; so as not materially to effect the outputindication.

If, in addition, the time constant of the servo response is madesufficiently greater than the natural period of oscillation of the vane,these latter relatively high frequency oscillations are not reproducedat the output shaft. There is thus achieved a performance in which theresponse time is uniform over the entire range of airspeeds, and isdetermined by the characteristics of the servo system only. The responsetime may be defined as the time taken for the angular displacement ofthe output shaft to reach a steady value in its damped oscillatoryresponse to a step change in the airflow direction. In practice, ofcourse, the damping provided by the tachogenerator feedback will benearly critical damping.

What we claim is:

1. An airflow direction indicator which includes an enclosure, a hollowshaft mounted for sealed rotation into and with respect to theenclosure, an inner shaft passing coaxially through the hollow shaft forrotation relative thereto, a vane arranged for location in the airstreamand rigidly attached to the inner shaft, a seal between the inner shaftand the hollow shaft arranged to allow their relative rotation through alimited angle, servo control means operable to rotate the ho]- low shaftto maintain the relative positions of inner shaft and the hollow shaftwithin said limited angle, and output means operable to indicate theangular position of the hollow shaft relative to a datum position.

2. An indicator as claimed in claim 1 in which the servo control meanscomprise sensing means for detecting the direction of rotation of theinner shaft relative to the hollow shaft, and drive means arranged to beso controlled by the sensing means as to cause the appropriate rotationof the hollow shaft.

3. An indicator as claimed in claim 2 in which the sensing meanscomprises an electric switch having contacts carried by the inner shaftand the hollow shaft and operable to energize the drive means when therelative rotation of the two shafts attains a predetermined value lessthan said limited angle.

4. An indicator as claimed in claim 3 in which the electric switchcontacts carried by the hollow shaft are combined with stop memberspreventing further relative rotation of the two shafts.

1. An airflow direction indicator which includes an enclosure, a hollow shaft mounted for sealed rotation into and with respect to the enclosure, an inner shaft passing coaxially through the hollow shaft for rotation relative thereto, a vane arranged for location in the airstream and rigidly attached to the inner shaft, a seal between the inner shaft and the hollow shaft arranged to allow their relative rotation through a limited angle, servo control means operable to rotate the hollow shaft to maintain the relative positions of inner shaft and the hollow shaft within said limited angle, and output means operable to indicate the angular position of the hollow shaft relative to a datum position.
 2. An indicator as claimed in claim 1 in which the servo control means comprise sensing means for detecting the direction of rotation of the inner shaft relative to the hollow shaft, and drive means arranged to be so controlled by the sensing means as to cause the appropriate rotation of the hollow shaft.
 3. An indicator as claimed in claim 2 in which the sensing means comprises an electric switch having contacts carried by the inner shaft and the hollow shaft and operable to energize the drive means when the relative rotation of the two shafts attains a predetermined value less than said limited angle.
 4. An indicator as claimed in claim 3 in which the electric switch contacts carried by the hollow shaft are combined with stop members preventing further relative rotation of the two shafts. 